The present invention relates to austenitic nickel-chromium-iron base alloys having properties making them especially well suited for use in high temperature, high energy neutron irradiation environments, such as found in a liquid metal fast breeder reactor (LMFBR). More particularly the present invention relates to improved titanium modified austenitic stainless steel alloys for use in nuclear applications.
One of the prime objectives in the efforts to develop a commercially viable LMFBR has been to develop an alloy, or alloys, which are swelling resistant and have the required post irradiation mechanical properties for use as fuel cladding and/or use as ducts. The fuel cladding will see service in contact with flowing liquid sodium and have a surface temperature of about 400.degree. C. (.about.750.degree. F.) to 650.degree. C. (.about.1200.degree. F.). A duct surrounds each bundle of fuel pins and sees service at about 380.degree. C. (.about.715.degree. F.) to 550.degree. C. (.about.1020.degree. F.). These components will be exposed at the aforementioned elevated temperatures to fast neutron fluxes on the order of 10.sup.15 n/cm.sup.2.s (E&gt;0.1 MeV), and should be capable of performing adequately to fluences on the order of 2 to 3.times.10.sup.23 n/cm.sup.2 (E&gt;0.1 MeV).
Initially one of the prime candidate alloys for the LMFBR, especially for fuel cladding and ducts, was 20% cold worked AISI 316 steel, a solid solution austenitic steel (see Bennett and Horton, "Materials Requirements for Liquid Metal Fast Breeder Reactors," Metallurgical Transactions A, (Vol. 9A, February 1978, pp. 143-149)). The chemistry specification, and material fabrication steps for nuclear grade 316 fuel cladding are described in U.S. Pat. No. 4,421,572 filed on Mar. 18, 1982.
However, the 316 alloy undergoes a high degree of void swelling during extended exposure to fast neutron fluxes at the LMFBR operating temperatures. Extensive development efforts aimed at reducing the swelling by either modifications to alloy chemistry or fabrication methods have been undertaken. For example, U.S. Pat. No. 4,158,606 pertains to one of these efforts wherein it was concluded that a combination of silicon and titanium additions to solid solution austenitic alloys such as 316 stainless should provide improvements in swelling resistance. This patent also states that minor additions of zirconium also appear to aid in reducing void swelling.
U.S. Pat. No. 4,407,673 issued on Oct. 4, 1983, and based on an application filed on Jan. 9, 1980, describes an effort to provide enhanced swelling resistance by alloy chemistry modifications, including reducing the chromium and molybdenum contents, while increasing the nickel, silicon, titanium and zirconium contents of the 316 alloy.
In the aforementioned materials phosphorus was considered to be an impurity, and the phosphorus contents of the alloys were maintained below 0.02 weight percent.
In spite of the aforementioned extensive efforts swelling due to void formation, and related to phase instabilities, brought about by prolonged exposure to high fluences of fast neutrons at elevated temperatures, remain as areas where significant improvements are needed. The present inventors believe that they have found a new class of austenitic alloys possessing a combination of excellent swelling resistance as well as good post irradiation mechanical properties.